1. Field of the Invention
The present invention relates to a polypropylene-based resin composition for stretched film, a process for producing this resin composition, and a stretched film. More particularly, the present invention relates to a polypropylene-based resin composition for stretched film, having transparency and anti-blocking property maintained at practical levels and excellent rigidity and stretching processability, and small heat shrinkage percentage, to a process for producing said resin composition, and to a stretched film made of the resin composition.
2. Description of Related Arts
Polypropylene-based stretched films are widely used as packaging materials, and there are conventionally known methods of mixing polypropylenes having different physical properties for improving the physical properties and stretching processability of a polypropylene-based stretched film.
For example, methods of mixing polypropylenes having different molecular weights are known, and JP58-173141A discloses a method for producing a polypropylene-based resin composition enabling the production of a stretched material excellent in extrusion moldability and stretching property for extrusion stretching, comprising producing a propylene homopolymer or random copolymer having a melt flow index of 0.02 to 5 g/10 minutes and a propylene homopolymer or random copolymer having a melt flow index of 50 to 1000 g/10 minutes by means of polymerization.
Further, JP06-248133A discloses a propylene-based composition rich in extrusion moldability and excellent in balance between rigidity and impact resistance, composed of a polypropylene of high molecular weight having an intrinsic viscosity of 1.0 or more and an meso pentad fraction of 0.90 or more and a highly stereoregular polypropylene of relatively low molecular weight having an intrinsic viscosity of 0.1 to 0.8 and an meso pentad fraction of 0.93 or more, and.
However, for use as stretched films of the propylene-based resin compositions mentioned above, improvement of rigidity, heat shrinkage percentage and stretching processability has been desired.
An object of the present invention is to provide a polypropylene-based resin composition for stretched film, having a transparency and anti-blocking property maintained at a practical level, excellent rigidity and stretching processability, small heat shrinkage percentage, a process for producing said resin composition, and a stretched film made of the same.
Namely, the present invention relates to a polypropylene-based resin composition for stretched film, comprising:
20 to 98 parts by weight of a propylene-based polymer(A) having an intrinsic viscosity [xcex7]A measured intetralinat 135xc2x0 C. of 1.8 to 8 dl/g and a melting temperature TmA measured by a differential scanning calorimeter (herein-after, referred to as xe2x80x9cDSCxe2x80x9d) of 140 to 165xc2x0 C., and
2 to 80 parts by weight of a propylene-based polymer B having an intrinsic viscosity [xcex7]B measured in tetralin at 135xc2x0 C. of 0.8 to 1.7 dl/g and a melting temperature TmB measured by DSC of 150 to 170xc2x0 C., wherein the ratio of the melting temperature TmA of the propylene-based polymer(A) to the melting temperature TmB of the propylene-based polymer(B) (TmA/TmB) is less than 1, the ratio of the intrinsic viscosity [xcex7]A of the propylene-based polymer(A) to the intrinsic viscosity [xcex7]B of the propylene-based polymer(B) ([xcex7]A/[xcex7]B) is more than 1 and less than 10 (1 less than [xcex7]A/[xcex7]B less than 10), and the melt flow rate is from 0.1 to 20 g/10 minutes; a process for producing said resin composition; and a stretching film made of the same.
The present invention will be described in detail below.
The propylene-based polymer(A) used in the present invention is a propylene homopolymer or a propylene-based random copolymer. As the propylene-based random copolymer, propylene-based random copolymers obtained by copolymerizing propylene with ethylene and/or at least one comonomer selected from xcex1-olefins having 4 to 20 carbon atoms are mentioned.
Examples of the xcex1-olefin having 4 to 20 carbon atoms include, for example, 1-butene, 2-methyl-1-propene, 1-pentene, 2-methyl-1-butene, 3-methyl-1-butene, 1-hexene, 2-ethyl-1-butene, 2,3-dimethyl-1-butene, 2-methyl-1-pentene, 3-methyl-1-pentene, 4-methyl-1-pentene, 3,3-dimethyl-1-butene, 1-heptene, methyl-1-hexene, dimethyl-1-pentene, ethyl-1-pentene, trimethyl-1-butene, methylethyl-1-butene, 1-octene, methyl-1-pentene, ethyl-1-hexene, dimethyl-1-hexene, propyl-1-heptene, methylethyl-1-heptene, trimethyl-1-pentene, propyl-1-pentene, dimethyl-1-butene, 1-nonene, 1-decene, 1-undecene, 1-dodecene and the like, preferably 1-butene, 1-pentene, 1-hexene and 1-octene, and more preferably 1-butene and 1-hexene.
As the propylene-based random copolymer(A), for example, a propylene-ethylene random copolymer, propylene-xcex1-olefin random copolymer and the like are listed. Examples of the propylene-xcex1-olefin random copolymer include, for example, a propylene-1-butene random copolymer, propylene-1-hexene random copolymer, propylene-1-octene random copolymer, propylene-ethylene-1-butene random copolymer, propylene-ethylene-1-hexene random copolymer, propylene-ethylene-1-octene random copolymer and the like, and preferably a propylene-ethylene random copolymer, propylene-1-butene random copolymer, propylene-1-hexene random copolymer, propylene-ethylene-1-butene random copolymer and propylene-ethylene-1-hexene random copolymer.
When the propylene-based random copolymer(A) is a propylene-ethylene random copolymer, the ethylene content is preferably 4% by weight or less, more preferably 3.5% by weight or less, further preferably 3% by weight or less, from the standpoint of the rigidity of the resulting polypropylene-based stretched film.
When the propylene-based random copolymer(A) is a propylene-xcex1-olefin random copolymer, the xcex1-olefin content is preferably 15% by weight or less, more preferably 12% by weight or less, further preferably 8% by weight or less, from the standpoint of the rigidity of the resulting polypropylene-based stretched film.
When the propylene-based random copolymer(A) is a propylene-ethylene-xcex1-olefin random copolymer, the total content of ethylene and xcex1-olefin is preferably 15% by weight or less, more preferably 12% by weight or less, further preferably 8% by weight or less, from the standpoint of the rigidity of the resulting polypropylene-based stretched film.
The intrinsic viscosity [xcex7] measured in tetralin at 135xc2x0 C. of the propylene-based polymer(A) used in the present invention is from 1.8 to 8 dl/g, preferably 1.9 to 6 dl/g. When the intrinsic viscosity [xcex7] of the propylene-based polymer(A) is less than 1.8 dl/g, the stretching processability of the resulting polypropylene-based resin composition may deteriorate, and when over 8 dl/g, the flowability in extrusion processing of the resulting polypropylene-based resin composition may deteriorate, and granule structures may tend to generate.
The melting temperature Tm measured by DSC of the propylene-based polymer A used in the present invention is from 140 to 165xc2x0 C., more preferably from 143 to 164xc2x0 C., further preferably from 147 to 163xc2x0 C. When the melting temperature Tm of the propylene-based polymer(A) is less than 140xc2x0 C., the rigidity of the resulting polypropylene-based stretched film may be deficient, and when over 165xc2x0 C., the stretching processability of the polypropylene-based resin composition for stretched film may deteriorate. Herein, the melting temperature Tm is determined from the peak temperature of a melting curve measured by DSC.
The meso-pentad fraction of the propylene-based polymer (A) used in the present invention is preferably from 0.7 to 0.99, more preferably from 0.8 to 0.98, from the standpoint of the rigidity of the resulting polypropylene-based stretched film.
The 20xc2x0 C. xylene soluble part (hereinafter, abbreviated as xe2x80x9cCXSxe2x80x9d) of the propylene-based polymer(A) used in the present invention is preferably 10% by weight or less, more preferably 6% by weight or less, from the standpoint of the rigidity and anti-blocking property of the resulting polypropylene-based stretched film.
The propylene-based polymer(B) used in the present invention is a propylene homopolymer or a propylene-based random copolymer, and preferably a propylene homopolymer. When the propylene-based polymer(B) is a propylene-based random copolymer, propylene-based random copolymers obtained by copolymerizing propylene and ethylene and/or at least one comonomer selected from xcex1-olefins having 4 to 20 carbon atoms are mentioned. As the xcex1-olefin having 4 to 20 carbon atoms, the same examples as for the above-mentioned xcex1-olefin having 4 to 20 carbon atoms in the propylene-based random polymer(A) are listed, and as the propylene-based random copolymer used as the propylene-based polymer(B), the same examples as for the above-mentioned propylene-based random copolymer used as the propylene-based polymer(A) are listed.
When the propylene-based polymer(B) is a propylene-ethylene random copolymer, the ethylene content is preferably 2% by weight or less, more preferably 1.5% by weight or less, further preferably 1% by weight or less, from the standpoint of the rigidity of the resulting polypropylene-based stretched film.
When the propylene-based polymer(B) is a propylene-xcex1-olefin random copolymer, the xcex1-olefin content is preferably 6% by weight or less, more preferably 4% by weight or less, further preferably 3% by weight or less, from the standpoint of the rigidity of the resulting polypropylene-based stretched film.
When the propylene-based polymer(B) in the present invention is a propylene-ethylene-xcex1-olefin random copolymer, the total content of ethylene and xcex1-olefin is preferably 6% by weight or less, more preferably 4% by weight or less, further preferably 3% by weight or less, from the standpoint of the rigidity of the resulting polypropylene-based stretched film.
The intrinsic viscosity [xcex7] measured in tetralin at 135xc2x0 C. of the propylene-based polymer(B) is from 0.8 to 1.7 dl/g, preferably 0.85 to 1.65 dl/g, more preferably 0.90 to 1.6 dl/g. When the intrinsic viscosity [xcex7] of the propylene-based polymer B is less than 0.8 dl/g, the stretching property of the resulting polypropylene-based resin composition for stretched film may deteriorate, and when over 1.7 dl/g, the rigidity of the resulting polypropylene-based stretched film may be deficient.
The melting temperature Tm measured by DSC of the propylene-based polymer(B) used in the present invention is from 150 to 170xc2x0 C., preferably from 155 to 167xc2x0 C., more preferably from 160 to 166xc2x0 C. When the melting temperature Tm of the propylene-based polymer(B) is less than 150xc2x0 C., the rigidity of the resulting polypropylene-based stretched film may be deficient, and usually, production of a propylene-based polymer having a melting temperature Tm of over 170xc2x0 C. is difficult.
The meso-pentad fraction of the propylene-based polymer(B) used in the present invention is preferably from 0.7 to 0.99, more preferably from 0.8 to 0.99.
The CXS of the propylene-based polymer(B) is preferably 4% by weight or less, more preferably 3% by weight or less, from the standpoint of the rigidity and anti-blocking property of the resulting polypropylene-based stretched film.
The content of the propylene-based polymer(A) in the polypropylene-based resin composition of the present invention is from 20 to 98 parts by weight, and the content of the propylene-based polymer(B) is from 2 to 80 parts by weight. Herein, the total amount of the propylene-based polymers(A) and (B) is 100 parts by weight. Preferably, the content of the propylene-based polymer(A) is from 25 to 96 parts by weight, and more preferably, the content of the propylene-based polymer(A) is from 30 to 94 parts by weight.
When the content of the propylene-based polymer(A) is less than 20 parts by weight, the stretching processability of the polypropylene-based resin composition may deteriorate, and when the content of the propylene-based polymer A is over 98 parts by weight, the rigidity of the polypropylene-based stretched film may be deficient.
The ratio of the melting temperature TmA of the propylene-based polymer(A) to the melting temperature TmB of the propylene-based polymer(B) (TmA/TmB) in the polypropylene-based resin composition of the present invention is less than 1, preferably 0.99 or less, more preferably 0.98 or less. When the ratio of the melting temperature TmA to the melting temperature TmB(TmA/TmB) is 1 or more, the rigidity of the resulting polypropylene-based stretched film may be deficient.
The ratio of the intrinsic viscosity [xcex7]A of the propylene-based polymer(A) to the intrinsic viscosity [xcex7]B of the propylene-based polymer(B) ([xcex7]A/[xcex7]B) in the polypropylene-based resin composition of the present invention is more than 1 and less than 10 (1 less than [xcex7]A/[xcex7]B less than 10), preferably from 1.2 to 9, more preferably from 1.4 to 8. When the ratio of the intrinsic viscosity [xcex7]A to the intrinsic viscosity [xcex7]B ([xcex7]A/[xcex7]B) is 1 or less, the rigidity of the polypropylene-based stretched film may be deficient, and when 10 or more, the flowability in extrusion processing of the resulting polypropylene-based resin composition may deteriorate, and granule structures may tend to generate.
The melt flow rate of the polypropylene-based resin composition of the present invention is from 0.1 to 20 g/10 minutes, preferably from 0.5 to 15 g/10 minutes, more preferably from 1 to 10 g/10 minutes. When the melt flow rate of the polypropylene-based resin composition is less than 0.1 g/10 minutes, the melt viscosity of propylene in a molten state is too high and the flowability in extrusion processing may be deficient, and when over 20 g/10 minutes, the formability such as stretching processability and the like may deteriorate.
The melting temperature Tm of the polypropylene-based resin composition for stretched film of the present invention is preferably from 145 to 166xc2x0 C., more preferably from 150 to 164xc2x0 C., further preferably from 155 to 163xc2x0 C. from the standpoint of simultaneous manifestation of particularly excellent stretching processability and rigidity, and a small heat shrinkage percentage.
The CXS of the polypropylene-based resin composition for stretched film of the present invention is preferably 4% by weight or less, more preferably 3.5% by weight or less, further preferably 3% by weight or less, from the standpoints of simultaneous manifestation of particularly excellent stretching processability and rigidity and a small heat contraction percentage, and of anti-blocking property.
As the process for producing the polypropylene-based resin composition for stretched film of the present invention, there are a method in which a propylene-based polymer(A) and a propylene-based polymer(B) are separately produced by individual polymerization, and are mixed, and a method in which using a two or more multi-stage polymerization method, and a propylene-based polymer(A) and a propylene-based polymer(B) in any stage, for example, a propylene-based polymer(A) are produced in the first step, or the second or latter steps and a propylene-based polymer(B) are produced in the second or latter steps, or the first step, respectively.
The method for each individual polymerization of a propylene-based polymer(A) and a propylene-based polymer(B) in the method comprising each individual polymerization of a propylene-based polymer(A) and a propylene-based polymer(B) and mixing of the polymer(A) and the polymer B obtained by each individual polymerization, is not particularly restricted, and known polymerization methods are mentioned. For example, there are a solvent polymerization method conducted in the presence of an inert solvent, a bulk polymerization method conducted in the presence of a liquid monomer, a gas phase polymerization method conducted in the substantial absence of a liquid medium, and the like. Preferable is a gas phase polymerization method. Further, polymerization methods combining two or more of the above-mentioned polymerization methods, and a two or more multi-stage polymerization method and the like are also mentioned.
The process for mixing the propylene-based polymer(A) and the propylene-based polymer(B) obtained by each individual polymerization is not particularly restricted, and methods of dispersing these polymer(A) and polymer(B) uniformly may be permissible. For example, there are a method in which a polymer(A) and a polymer(B) are mixed by a ribbon blender, Henschel mixer, tumbler mixer or the like, and the mixture is melt-kneaded by an extruder or the like, a method in which a polymer(A) and a polymer(B) are each individually melt-kneaded to be palletized, and the palletized polymer(A) and polymer (B) are mixed by the same manner as described above, and further melt-kneaded, a method in which a polymer(A) and a polymer(B) are each individually melt-kneaded to be palletized, and the palletized polymer(A) and polymer(B) are blended by dry blend or the like, then, mixed directly by a film forming machine, a method in which a polymer(A) and a polymer(B) are each individually melt-kneaded to be palletized, and the palletized polymer(A) and polymer(B) are each individually fed through an extruder of a film forming machine to be mixed, and other methods. Further, there are also exemplified a method in which a master batch containing 1 to 99 parts by weight of a propylene-based polymer(A) based on 100 parts by weight of a propylene-based polymer(B) is previously produced, and appropriately mixed so as to provide a given concentration, and the like.
Furthermore, in mixing the propylene-based polymer(A) and the propylene-based polymer(B) obtained by each individual polymerization, stabilizers, lubricants, antistatic agents, anti-blocking agents, inorganic or organic fillers and the like may also be added, in amounts not deteriorating the object and effect of the present invention.
The method for producing a propylene-based polymer(A) and a propylene-based polymer(B) by means of a two or more multi-step polymerization, is not particularly restricted, and known polymerization methods are mentioned. For example, there are a method in which a solvent polymerization method conducted in the presence of an inactive solvent, a block polymerization method conducted in the presence of a monomer in the form of liquid, a gas phase polymerization method conducted in the substantial absence of a medium in the form of liquid, and other methods are optionally used in combination of two or more and a propylene-based polymer(A) and a propylene-based polymer(B) are polymerized in any of these stages, and other methods.
The polypropylene resin composition obtained by the multi-stage polymerization method may further mixed, and as the process for further mixing, a method of melt-kneading by an extruder or the like, and the like are listed. In the mixing, additives and fillers as described above may also be added.
As the polymerization catalyst used for production of a propylene-based polymer(A) and a propylene-based polymer(B) used in the present invention, a catalyst for stereoregular polymerization of propylene is used both in the case of each individual polymerization of them and in the case of use of the multi-stage polymerization method.
The catalyst for stereoregular polymerization of propylene includes, for example, a titanium trichloride catalyst, catalyst systems prepared by combining an organoaluminum compound and optionally a third component such as an electron donative compound, with a solid catalyst component such as a Tixe2x80x94Mg-based catalyst containing titanium, magnesium, halogen and electron donor as essential components, metallocene-based catalysts and the like.
Preferable are solid catalyst components containing magnesium, titanium, halogen and electron donor as essential components, and catalysts obtained by combining an organoaluminum compound and an electron donative compound, and specific examples thereof include catalyst systems described in U.S. Pat. Nos. 5,608,018, 4,743,665 and 4,672,050.
The methods of forming and stretching the polypropylene-based composition stretched film of the present invention are not particularly restricted, and a uniaxial stretching and biaxial stretching, that is, generally, a machine direction uniaxial stretching, transverse direction uniaxial stretching, sequential biaxial stretching, simultaneous biaxial stretching, tubular biaxial stretching and the like are listed. These stretching methods are described below.
Machine Direction Uniaxial Stretching
A polypropylene is melted by an extruder, then, extruded through a T die, and solidified in the form of sheet by cooling with a cooling roller. Then, the resulted sheet is pre-heated and stretched in the machine direction by a series of heating rolls, and if necessary, subjected to a corona treatment or the like, and wound.
Transverse Direction Uniaxial Stretching
A polypropylene is melted by an extruder, then, extruded through a T die, and solidified in the form of sheet by cooling with a cooling roller. Then, both ends of the resulted sheet are clamped by two lines of chucks arranged along the flow direction, and stretched in the transverse direction by spreading the interval of the above-mentioned two lines of chucks in a heating furnace composed of a pre-heating part, stretching part and heat treatment part, and if necessary, subjected to a corona treatment or the like, and wound.
Sequential Biaxial Stretching
A polypropylene is melted by an extruder, then, extruded through a T die, and solidified by cooling with a cooling roll. Then, the resulted sheet is pre-heated and stretched in the machine direction by a series of heating rolls. Subsequently, both ends of the resulted sheet are clamped by two lines of chucks arranged along the flow direction, and stretched in the transverse direction by spreading the interval of the above-mentioned two lines of chucks in aheating furnace composed of a pre-heating part, stretching part and heat treatment part, and if necessary, subjected to a corona treatment or the like, and wound.
The fusion temperature of polypropylene in the sequential biaxial stretching is usually from 230 to 290xc2x0 C. The machine direction stretching temperature is usually from 130 to 150xc2x0 C., and the machine direction stretching magnification is usually from 4 to 6. The transverse stretching temperature is usually from 150 to 165xc2x0 C., and the transverse stretching magnification is usually from 8 to 10.
Simultaneous Biaxial Stretching
A polypropylene is melted by an extruder, then, extruded through a T die, and solidified by cooling with a cooling roller. Subsequently, both ends of the resulted sheet are clamped by two lines of chucks arranged along the flow direction, and stretched in the machine direction and transverse direction simultaneously by spreading the interval of the above-mentioned two lines of chucks and the interval between chucks in individual line in a heating furnace composed of a pre-heating part, stretching part and heat treatment part, and if necessary, subjected to a corona treatment or the like, and wound.
Tubular Biaxial Stretching
A polypropylene is melted by an extruder, then, extruded through an annular die, and solidified in the form of tube by cooling in a water tank. Then, the resulted tube is pre-heated with a heat furnace or a series of heat rolls, then, passed through low speed nip rolls, and wound with high speed nip rolls to be stretched along the flow direction. In this operation, the tube is stretched also in the transverse direction, by swelling the tube with the action of internal pressure of air accumulated between the low speed nip rolls and the high speed nip rolls. The stretched film passed through the high speed nip rolls is thermally treated by a heating furnace or series of heat rolls, and if necessary, subjected to a corona treatment or the like, and wound.